I i
XMM-Newton Archival Study of the ULX Population in Nearby Galaxies
Lisa M. Winter
Astronomy Department, University of Maryland, College Park, MD 20742
IwinterQastro .umd. edu
Richard Mushotzky
Goddard Space Flight Center, Greenbelt, MD 20771
Christopher S. Reynolds
Astronomy Department, University of Maryland, College Park, MD 20742
ABSTRACT
We have conducted an archival XMM-Newton study of the bright X-ray point sources in 32 nearby galaxies. From our list of approximately 100 point sources, we attempt to determine if there is a low-state counterpart to the Ultraluminous X-ray (ULX) population. Indeed, 16 sources in our sample match the criteria we set for a low-state ULX, namely, L,! > ergs-’ and a spectrum best fit with an absorbed power law. Further, we find evidence for 26 high-state ULXs which are best fit by a combined blackbody and a power law. As in Galactic black hole systems, the spectral indices, I’, of the low-state objects, as well as the luminosities, tend to be lower than those of the high-state objects. The observed range of blackbody temperatures is 0.1-1 keV with the most luminous systems tending toward the lowest temperatures. We also find a class of object whose properties (luminosity, blackbody temperature, and power law slopes) are very similar to those of galactic stellar mass black holes. In addition, we find a subset of these objects that can be best fit by a Comptonized spectrum similar to that used for Galactic black holes in the “very high” state, when they are radiating near the Eddington limit.
Subject headings: galaxies: general - surveys - X-rays:binaries - accretion, accretion discs 4
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1. Introduction
Through X-ray observations of nearby galaxies, a class of Ultraluminous X-ray (ULX) sources has emerged. These are pointlike, non-nuclear sources with bolometric luminosities in excess of the Eddington limit for a 20 Ma black hole, or Lbol > 2.8 x lo3’ ergs-’. The true nature of these sources is unclear, and this class more likely includes several different types of objects. Though a number of these sources are located within a few parsecs of their host galaxy’s dynamical center, they do not exhibit many of the characteristics of active galactic nuclei (AGN). Because the ratio of X-ray to optical flux is a factor of 10 greater than that of AGN (Anderson et al. 2003; Stocke et al. 1983)) these objects are fairly easy to recognize in X-ray imaging data. Assuming that the Eddington limit is obeyed by black hole accretion, the existence of such luminous non-AGN sources presents a puzzle. Several models have been proposed to account for the high luminosities of the ULXs. Among these are relativistic and non-relativistic beaming from stellar-mass black hole systems (Kording et al. 2002) and accretion of matter into intermediate mass black holes (IMBH). In several systems (NGC 1313 X-2, M81 X-9, etc.), detection of emis- sion nebulae surrounding the ULX supports isotropic emission from the central source (Pakull & Mirioni 2003), which cannot be described through relativistic beaming. Further, a number of ULX (NGC1313 X-1, etc.) X-ray spectra are best fit with combined multi-component blackbody (MCD) and power law fits, similar to Galactic black holes in their high-state. Recently, Miller et al. (2003) firid that rriariy spectral fits of ULX require cool accretion disk temperatures of ap- proximately 100 eV. The theoretical relationship between black hole mass and disk temperature (T cx M-1/4) has been observed to hold true for stellar mass (typically around 1keV) and supermas- sive (around 10-100eV) black holes (Makishima et al. 2000). Thus, the cool accretion disk ULXs would correspond to a populat.ion of high-state IMBHs with masses of M 16 - lo4 Ma. If some ULXs do indeed represent a class of high-state IMBHs, similar to the high-state stellar mass black holes in our galaxy, we might also expect to see the low-state objects from this same population. In Galactic black hole systems, the low-state is generally characterized by lower luminosity, with L< 0.1 L~dd(Done & Gierlinski 2003), and a power law spectrum, typically with index I? M 2.0 (McClintock & Remillard 2004). In this study we seek to find these low-state sources, classify the properties of both high-state and low-state ULXs, and examine whether these data are consistent or inconsistent with the IMBH hypothesis.
We present .the results of a detailed analysis of ULXs in nearby galaxies observed with the European Space Agency’s XMM-Newton observatory. Only XMM-Newton provides’the count rates and bandpass necessary to distinguish different spectral models for the ULX and accurately deter- mine both the temperature of the thermal component expected for high-state objects and whether this component is required in the spectral modeling of these objects.
In Section 2, we detail the observations examined from the XMM-Newton archives and explain the data analysis for the individual point sources. In Sections 3 and 4, we discuss the spectral fitting technique as well as simulations we conducted to determine their validity. We discuss the implications of our results in Section 5.
2. Observations and Data Reduction
The data used in this investigation were drawn from the XMM-Newton public data archive. Assuming that low-state ULXs exist in the luminosity range of 1038-39ergs-’, we conducted simu- lations to determine the optimum criteria for observations capable of resolving point sources of this luminosity. This luminosity range was chosen on the assumption that an approximately 100 Ma black hole would radiate at = 5% of the L~ddin the low-state (Done k Gierlinski 2003). Our simulations sought to determine the number of photons required to distinguish between spectral fits using a power law model, a bremsstrahlung model, and a combined blackbody and power law
model at an Lx N ergs-’, in addition to limiting the possibility of source confusion. We adopt these models since they qualitatively correspond to the spectra of low-state black hole X- ray binaries, neutron star X-ray binaries, and high-state black hole X-ray binaries, respectively. In order to distinguish between the different spectral fits for objects with Lx - ergs-I, our selection includes galaxies that were observed for at least 10 ks (with the exception of the bright ULX in NGC 5408, which had enough photons for analysis despite the low exposure time) with XMM-Newton and that are no more distant than 8 Mpc. This yields a minimum of 400 counts, objects with Ls > 2 x 103*ergs-l.
. Our sample of galaxies is selective in that it represents objects of interest in the X-ray band. We include details on these host galaxies in Table 1. In most cases, the original choice to observe these galaxies (and hence their inclusion in our sample) was unlikely to be influenced by any previous knowledge of their ULX population. Hence, this sample should not be biased in terms of the ULX population. Our galaxies include only spirals and irregulars. Figure 1 displays the distribution of galaxy type. We reduced the data using the XMM-Newton Science Analysis System (SAS) version 6.0.0. Since the processed pipeline products (PPS) were created with earlier versions of SAS, the ob- servation data files (ODF) were used to produce calibrated photon event files for the EPIC-MOS and PN cameras using the commands ernchain and epchain. Following this, the events tables were filtered using the standard criteria outlined in the XMM ABC Guide. For the MOS data (both MOSl and MOS2 cameras), good events constitute those with a pulse height in the range of 0.2 to 12 keV and event patterns that are characterized as 0-12 (single, double, triple, and quadruple pixel events). For the PN camera, only patterns of 0-4 (single and double pixel events) are kept, with the energy range for the pulse height set between 0.2 and 15 keV. Bad pixels and events too close to the edges of the CCD chips were rejected using the stringent selection expression “FLAG == 0”. Time filtering was applied as needed by editing the light curve produced in zmmselect for the entire obscrvation. Flare events (distinguished by their high count rate) detected in all three cameras, were cut using the tabgtigen task as outlined in the ABC Guide. Such filtering was only done as needed. 1 I a
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Before extracting spectra of the brightest sources, contour maps of the X-ray observation were overlaid on Digital Sky Survey (DSS) images. This ensured that bright foreground stars and background AGN were easily distinguished, and thereby not included in the spectral fitting. Also, we checked the XMM-Newton positions with NED and SIMBAD to determine if they coincide with any known background galaxies or QSOs. A list of these bright fore-ground or background sources is included in Table 7.
3. Spectral Fitting
Spectra for the bright point sources were extracted using the SAS task especget. With this task we created spectra (for both the source and background), response matrices, and ancillary response files for all three EPIC cameras, when possible. The typical extraction radius was 20 arcseconds, but depending on both the size and proximity of a source to another source, the extraction radius ranged from 9 - 87 arcseconds. Background spectra were extracted either in an annulus centered on the source, or in a circle of appropriate size away from the source, depending on the proximity of the central source to other X-ray sources. Once the spectra were obtained, they were rebinned to require at least 20 counts per bin, using the command grppha in LHEASOFT. The list of sources, with position and count information, is included in Table 6. The extracted spectra were fit with standard models in XSPEC v11.3.1. For each source, we fit the PN and MOS spectra simultaneously in the 0.3-10 keV range. We allowed a free normalization constant to account for the differences in flux calibration between the three cameras (similar to Jenkins et, al. (2004)). Each source was first fit with an absorbed single component model. In all cases we used the standard absorption model wabs, leaving the column density as a free parameter. Results of the single-component fits are seen in Table 2. We include in this table only the best-fit parameters for those sources best described by a single-component model. The flux values quoted represent the unabsorbed flux in the PN spectra, in the 0.3-10 keV band. All errors quoted, here and subsequently, correspond to the 90% confidence level for one degree of freedom (Ax2 = 2.71). The luminosities were calculated from the unabsorbed flux using the distances quoted in Table 1. Both flux and luminosity correspond to those of the best fit model (power law or bremsstrahlung). It should be noted that since our selection criteria was based on a count rate cutoff, due to the variety of spectral forms, the inferred luminosity cutoff will not be uniform.
For a number of sources, the single-component models did not adequately describe their spec- tra. For these sources we employed an absorbed two-component blackbody and power law model. In Table 3 we present the results of the sources best fit by this two-component fit. We include the improvement in x2 of the two-component fit over the simple power law. We include the power law best fits to these sources in the appendix. Table 4 includes those sources where both the single-component and two-component models were indistinguishable. Furthcr, for those sources we classify as ULXs (see section 5 for our criteria), we computed -5- bolometric luminosities. We used the exponentially cutoff power law spectrum of Magdziarz & Zdziarski (1995), model pezruv in XSPEC, with a cutoff energy of 10 keV and the reflection pa- rameter set to zero. This model was used in place of the power law component. We computed an unabsorbed flux in the 0.1 - 100 keV range through use of the dummyresp command (which extends the model beyond the observation's energy range). The luminosity was then computed using the distances listed in Table 1. We quote these values as Lupperin Table 5. We note that these values represent an upper limit on the bolometric luminosity for steep power law (I' > 2) objects, since we would expect the power law component to cutoff at some low energy. However, for flat spectrum (I' < 2) sources Lbol is a lower limit. For our ULX sources modeled by a combined blackbody and exponentially cutoff power law, we estimate a more accurate Lbol, calculated from the flux in the range of 2x kT- 100 keV where kT is the blackbody temperature obtained from the model. In galactic X-ray binary systems, the power law component of the X-ray spectrum is believed to be from Comptonization in a corona. The photons supplying this energy originate from the blackbody continuum emanating from the accretion disk. Thus, a natural cutoff for this power law component occurs at the peak emission of the blackbody (which is approximately 3x kT). These estimated blackbody values are within 95% of the full integrated blackbody flux and are therefore a good approximation to the data. We note that our bolometric luminosities, on average, are a factor of 1.08 greater than the X- ray luminosities in the 0.3- 10 keV band for the objects best fit by a combined blackbody and power law. Thus, to good approximation, the X-ray luminosity is the bolometric luminosity. However, for the objects best fit by a simple power law, the average bolometric luminosity is roughly a factor of 7 greater than the X-ray luminosity in our band. This average is dominated by the steep power law objects, in particular Holmberg I1 XMMl (I' = 3.09). Excluding this object, we get an average bolometric luminosity that is 2.8 times the X-ray flux and more indicative of the general properties of these power law-fit objects.
In this large sample of point sources, we came across a number of objects whose spectra were not well fit by the standard models employed. We briefly describe these sources in the appendix.
4. Spectral Simulations
In order to determine whether the blackbody component is statistically significant for the sources fit with a two-component model, we simulated spectra of some of the brightest sources. Table 8 shows the results of these simulations. We simulated spectra using the XSPEC command fakeit for 10 bright (> 5000 counts) sources that were best fit by a combined blackbody and a power law model. The fukeit command uses Monte Carlo simulations in order to create a simulated spectrum from the original dataset. We simulated spectra with both 10% and 5% of the actual exposure time. These spectra were then fit with the three standard spectral fits employed in this study. .
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Comparing chi-square values for the simulated spectral fits, the combined blackbody and power law fit is the best fit for all simulated spectra at 10% the actual exposure time, corresponding to a source that is ten times weaker. At 5% the exposure time, 70% of the simulated spectra are well-fit by the combined model. These results suggest that indeed the blackbody component is statistically significant and distinct from a pure absorbed power law model. Thus, we are confident that the best fit for the bright combined blackbody and power law spectral fits is a valid description of the data.
5. Discussion
We have determined best-fit spectral parameters of the bright X-ray sources in 32 nearby galaxies. In choosing three “standard” models for our study, we hoped to accurately separate high and low state ULXs from other bright X-ray sources. We specifically chose to fit the data with the bremsstrahlung model in order to identify neutron star X-ray binaries within our sample. We cross-referenced the X-ray positions of our sources with both NED and SIMBAD in order to identify known supernovae, galaxies, and stars. In addition, we examined the DSS optical images to place the position of our sources within their respective galaxies. Such analysis aimed to minimize contamination of our sample of ULXs with bright background and foreground sources. Further, we examined XMM-Newton’s Optical Monitor data in the visual bands (U, B, V). The PPS contain point source detection files for the OM data. We overlaid these point source detections with X-ray contour maps in order to determine the brightest possible optical count rates for the X-ray sources, which were then converted into fluxes using the OM calibration documentation. In Figure 2, we plot the distribution of the logarithm of the X-ray to optical flux for the brightest possible optical counterpart inside the XMM-Newton error circle. Only 13 of the 32 host galaxies had visible OM data during the observations. Of these 13 galaxies, 40 sources were in the range of the OM data and only 14 were coincident with an optical point source. Therefore, the majority of our sources have X-ray/optical flux ratios that are larger than those displayed. We estimate the point source detection limit of the OM U filter as approximately 1.24 x ergcm-2 s-’. For an unabsorbed X-ray flux of 1.0~ ergcm-2 s-l, this corresponds to log(f,/foPt) = 1.9. Therefore, the average value for our sources should fall around 2 or greater. The average distribution for QSOs and AGN centers around 0 and 0.8 for BL Lacs (Anderson et al. 2003). Thus, our objects have ratios of L,/L,,t atleast 10 times higher than those of AGN and 100 times greater than stars. Recently, Gutierrez & Lopez-Corredoira (2005) identify six ULXs from the catalog of Colbert & Ptak (2002) as QSOs. They hypothesize that a large number of ULXs may in fact be quasars at higher redshift than their supposed host galaxy. However, unlike the objects studied in Gutierrez & Lopez-Corredoira (2005), our ULX sources are all spatially coincident with thc optical host galaxy. In addition. a majority of our ULXs are not in the proximity of a cataloged optical point source. The X-ray/optical flux ratios of our sources are much larger, on average, than might be expected for -7- a &SO. It is also worth noting that while some cataloged ULXs may be QSOs, optical identifications have been made associating other ULXs with a type B supergiant companion (Kuntz et al. 2005; Liu et al. 2004).
5.1. Classification Criteria
The spectral fits indicate that we can indeed distinguish a class of low-state ULXs from the high-state objects. As a constraint on our ULX classification, we require our X-ray sources to clearly coincide with the optical extent of the host galaxy (as determined from the DSS images). Of the sources in Table 2, 16 are “low-state” objects, having unabsorbed luminosities > lo3” erg s-’ and spectra that are best fit by power law models. Further, 26 sources have unabsorbed Lx > 3 x ergs-’ and spectra that are best fit by combined blackbody and power law models. These are “high-state” objects. In addition to these high and low state ULXs, we find a large number of sources best fit by a combined blackbody and power law model but below our threshold of LX = 3 x lo3’ ergs-’. Many of these sources may be accreting stellar mass black holes. Some of these sources were found away from the optical extent of the targeted galaxy (from our analysis of the DSS images), and may represent background AGN.
5.2. Low-State ULX
For Galactic black hole X-ray binaries, spectral indices of low-state objects are typically lower than those of high-state objects (McClintock & Remillard 2004). In Figure 3, we plot the distri- bution of the spectral index for both high-state and low-state objects. As in the Galactic sources, it is clearly shown that the spectral indices of the high-state objects are indeed larger. Of further interest, the distribution of spectral index for low-state objects looks remarkably similar to the distribution of spectral index for moderate luminosity quasars, many of which are thought to be the analogs of low-state black holes (Porquet et al. 2004). This supports the classification of these objects as accreting black holes.
The low hard X-ray state of X-ray binaries is associated with a low accretion rate from the companion object. Therefore, on average, we expect the luminosities of the low-state objects to be lower than the high-state objects. Figure 4 displays luminosity as a function of the spectral index. On average, the highest luminosity low-state objects have luminosities lower than those of the high-state objects. We find mean values of I? = 2.46, with a root mean square (rms) deviation of S= 0.12, and Lx = 1.4 x 1040ergs-1, log(S) = 1.6, for the high-state objects. This calculation excludes the 3 objects with spectral indices greater than 3.5. For the low-state objects, we find mean values of I? = 2.09, with arms deviation of S= 0.10, and LX = 2.2 x lo3’ ergs-’, log(S) = 2.1. The lower Ls values of the low-state objects imply that they may indeed be accreting at a lower rate than the high-state objects. This can further be seen in the bolometric luminosities -8- listed in Table 5. If these objects are accreting at a rate similar to galactic low-state black holes (0.1 x L~dd)(Done & Gierlinski 2003), we can estimate their masses as
with L~ddas the Eddington luminosity for a 1Ma object (1.3 x lo3’ ergs-I). Our mass estimations, based upon our limits to the bolometric luminosities, yield masses of 20 - 1524MD (see Table 5), precisely what we might expect for a population of IMBHs.
5.3. High-State ULX
If the high-state ULXs represent a class of intermediate mass black hole systems, their X-ray spectra should be best fit by a combined blackbody and power law model. Scaling for the mass of the black hole, we would expect a relationship of T 0; M-1/4 between black hole mass and blackbody temperature (Makishima et al. 2000). This would indicate a thermal component of - 100 eV. A few objects have been reported to display this property (Miller et al. 2003; Roberts & Warwick 2000). In Figure 5, we graph the distribution of the thermal component for our classified high-state objects. We find that there are two peaks in the distribution among the thermal component, one at approximately 100 eV and another centered close to 1 keV. This could indicate two different classes among the high-state objects. It is possible that those objects with blackbody components near 100 eV are indeed high-state intermediate mass black holes. The second peak, centered around 1 keV, has thermal components reminiscent of the Galactic black hole systems in our own galaxy. These systems may be stellar-mass black holes accreting matter at an increased rate. If this were the case, we would expect the luminosities of the sources exhibiting a higher blackbody temperature to be lower. In the second graph of Figure 5, we plot the relationship between blackbody temperature and Lx in the 0.3 - 10 keV band. Once again, two classes of ULXs are seen. The most luminous objects are those with low blackbody temperatures. On average, the less luminous sources exhibit higher blackbody temperatures. The second, low-luminosity, class of ULX is clearly distinguishable in both plots of Figure 5. We found that, with the exceptions of NGC 253 XMM1, M81 XMM1, and NGC 5204 XMM1, the spectra of these objects could be well-described by an absorbed Comptonization (compST) model (Sunyaev & Titarchuk 1980) used to fit galactic black holes in the “very high” state when they are radiating at the Eddington limit. This model simulates Compton scattering of cool photons on the hot electrons of a completely ionized plasma. We present the best-fit parameters for the Comptonization model in Table 10.
This “very high” state has been observed (Miyamoto et al. 1991) in a few Galactic black holes. McClintock & Remillard (2004) use the alternative nomenclature of the steep power law state, a state which is characterized by r > 2.4 and a luminosity which may or may not be greater than -9- the luminosity in the high-state. Yet another rubric for the very high state emerged in Kubota et al. (2001) and Kubota & Makishima (2004), where they identify this as the “anomalous” state, a state whose spectrum can be well fit by an inverse Compton scattering model. Regardless of the name, our best-fit Comptonization sources likely fit into this category. The luminosities of these sources suggest that they are stellar mass black hole systems in this anomalous/very high state. As with the low-state, we include mass estimates for our high-state objects in Table 5. We assume that the high-state objects are radiating at L~dd.We find masses of 1.6 - 38Mo for the sources well fit by the Comptonization model. The other high-state ULXs range from 16.5 - 1354 Ma, analogous to the low-state ULX masses computed.
5.4. Temperature Gap
In addition to the existence of ULXs with low blackbody temperatures, the temperature distri- bution (Figure 5, left panel) displays a “gap” which is of particular interest - there is a complete absence of objects with temperatures in the range 0.26 keV to 0.50 keV. It is tempting to take this as evidence for a gap in the mass distribution of these accreting black holes. Since, for a given luminosity) we expect the temperature to vary as T 0: L1/4M-1/2,this factor of two gap in the temperature distribution translates into a factor of four gap in the black hole mass distribution. If this result is borne out by further study, it provides an important clue to the origin and evolution of intermediate mass black holes. One popular idea is that intermediate mass black holes formed from the collapse of massive Population I11 stars (Madau & Rees 2001). Models suggest that Pop I11 stars with zero age main sequence (ZAMS) masses in the range 25-140Ma and above 260Ma collapse to produce black holes (Heger & Woosley 2002) whereas in the range of ZAMS masses 140-260Ma, pair-instability supernovae lead to the complete disruption of the stars (i.e.) no remnant black hole remains). Hence, this model for IMBH formation predicts a gap in the IMBH initial mass function in the range of approximately 60-200MQ (although this is uncertain on the low end due to the effect of the pulsational pair-instability on the pre-collapse core). One possibility is that the gap in our observed temperature distribution (and hence the inferred gap in the mass function) is due to this effect of the pair instability supernovae in Pop I11 stars. This would require that the current IMBH mass function is approximately the same as the initial IMBH mass function. In other words, it requires that most IMBHs (especially those just below the gap) have not grown significantly due to accretion since their formation and, hence, that the ULX phase represents a short fraction of the life-time of an IMBH (f << tsal/ta, where tsal == 45~0.1Myr is the e-folding timescale for Eddington limited black hole growth with radiative efficiency E = 0.1~o~). An alternative interpretation of the inferred mass gap is to suppose that two fundamentally different modes of formation lead to a strong bi-modality in the final black hole mass function. Black hole masses below the gap can be readily understood through normal stellar processes. A separate and distinct population of significantly more massive black holes may result from dynamical - 10 - processes in the core of dense globular clusters (Miller & Hamilton 2002; Gultekin, Miller, & Hamilton 2004).
5.5. Galactic HMXBs
Supposing that the Galaxy’s bright X-ray population is representative of low-redshift galaxies, we expected to find a number of sources similar to Galactic X-ray binaries in our sample. As previously stated, we set a luminosity cutoff of ~3 x lo3’ ergs-’ (0.3 - 10 keV band) in order to distinguish between galactic HMXBs and high-state ULXs. In our sample, we find approximately 24 sources with luminosities below our high-state ULX cutoff, X-ray positions within the optical extent of their host galaxy, and no obvious optical counterpart. The unabsorbed luminosities for these sources range from 0.4 - 2.5 x lo3’ erg s-l (0.3 - 10 keV band). Two of these sources were transients. Of the four host galaxies with multiple observations examined, two of these galaxies contained solely ULX sources in our luminosity regime (Holmberg I1 and NGC 5204). Each of the remaining two (NGC 253 and NGC 4258) had a transient source best fit by a combined blackbody and a power law. This suggests an interesting diagnostic in terms of distinguishing our ULX sources from a normal HMXB population. In our own galaxy, most HMXBs vary on timescales of days or less and most of the black holes in the Milky Way are transients. The figures in a recent paper of Kalogera et al. (2004), determined through detailed mass-transfer calculations, indicate that transient behavior should not be expected from a population of IMBHs (M. Coleman Miller 2005, private communication). Thus, on average, our ULX sources should remain X-ray bright in multiple observations. Through a literature search, we found that 37/42 ULX sources are distinguishable in ROSAT observations and thus are luminous for greater than 10 years and therefore are not transients. Examination of the long term light curves show that most sources vary by less than a factor of 3 over the timescale from ROSAT to XMM.
As a possible further diagnostic, we constructed a color-color diagram for our ULX sources. We adopted the colors of Done & Gierlinski (2003) in order to compare our sample with their sample of Galactic X-ray sources. Thus, our colors were constructed from unabsorbed model fluxes in four energy bands: 3-4, 4-6.4, 6.4-9.7, and 9.7-16 keV. The XSPEC command dummyresp was used to calculate a flux based on the model for the 10-16 keV range. We plot colors for a pure unabsorbed power law (from I’ = 1.5 - 3.0) and an unabsorbed MCD model (diskbb in XSPEC with kTi, = 5.0 - 0.2eV) for comparison. Comparing our Figure 6 with Figure 8 of Done & Gierlinski (2003), we find that our ULX sources largely lie along the same regions as their black hole sources. A few ULX sources, however, lie in the region occupied by atoll and Z-sources in the plot of Done & Gierlinski (2003). These sources were those best fit by a Comptonization model. I
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5.6. Galaxy Sample
It has been shown that the ULX population is proportional to the host galaxy's star formation rate (SFR) (Ranalli et al. 2002; Grimm et al. 2003). The far-infrared luminosity of a galaxy is used as an indicator of the SFR. In order to compare the ULX population of a galaxy with the SFR we followed a similar approach to Swartz et al. (2004). We calculate the FIR flux from observations taken by the Infrared Astronomical Satellite. As in Swartz et al. (2004), the flux between 42.4 and 122.5 pm is approximated as 1.26~10-~~(2.58S~o+S~oo)ergcm-2s-'. The values of the flux at 60pm (S~O)and 100 pm (Sloe) were obtained from either Ho et al. (1997) or NED. Luminosities were calculated using the distances quoted in Table 1. We list these values in addition to the number of ULXs observed in individual galaxies in Table 11. The number of ULXs includes both the objects we classify as high and low state ULX as well as those sources resolved by Chandra. In Figure 7, we show two plots relating the number of ULXs to LFIR. It has been suggested by Grimm et al. (2003) that the luminosity function in the X-ray regime from HMXBs is related to SFR. In our first plot, we find that the galaxies with the highest LFIR seem to have fewer ULXs than may be expected from the luminosity functions of Grimm et al. (2003). Thus, in a direct comparison, our results do not agree with their predictions.
The second plot displays the average number of ULXs/galaxy, binned according to luminosity. This plot is extremely similar to Fig. 15 of Swartz et al. (2004) for spiral galaxies. Thus, once again, it seems that the connection between SFR and the ULX population in spirals is supported. For irregular galaxies, however, there seems to be more of a spread in the distribution. This could be the result of poor sampling - most of the bins contain only one galaxy. Another possibility is that there is no direct correlation in irregular galaxies or that the overall star formation in these galaxies is less ordered or clumpier. If the latter is the case, the overall SFR of the galaxy is only an average over a wide range of values. We shall address this issue again in the next paper in this series (L.M. Winter et al., in preparation) where we discuss the local environment of the ULX in our sample. In Figure 8 we plot the distribution of column densities among the ULX. We subtracted the Galactic column density towards the galaxy (obtained from the nH FTOOL and listed in Table 1) from the values obtained through spectral fits. We note that on average the ULXs have large column densities. The typical Galactic column density along a line of sight is M 4 x lo2' cm-2. If the ULX is located on the opposite side of its host galaxy, we might expect maximum column densities of M 1.2 x lo2' cm-2. However, most of our sources have column densities well above this value. This may imply that the local environment of the ULXs contains an extra source of absorption. In order to better understand the relationship between SFR and the ULX population, it is necessary to extend ULX studies to other wavelengths. In particular, it becomes important to analyze UV and IR images close to the ULX. .
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6. Conclusion
We have found from our XMMsurvey of the ULX population in nearby galaxies that there exists a population of objects whose X-ray spectral properties closely match the low-state spectra of Galactic black holes, but whose luminosities lie in the range of Lbol M 2 x - 1 x 1040 ergs-’. In the Milky Way, black holes with these spectral properties radiate at only 0.05 of the Eddington limit. If this is also true for this population, it indirectly implies that these objects have a mass greater than M 30 Mo ranging up to 1500 Ma and thus should be IMBHs. The existence of such objects was “predicted” on the basis that the ULXs previously studied shared the X-ray spectral characteristics of high-state Galactic black holes; namely, an X-ray spectrum best fit by a combined blackbody and a power law (Miller et al. 2003), but with much higher luminosities. If these objects are high-state IMBHs, the corresponding low-state objects should also exist.
Our survey has also uncovered a large population of objects whose X-ray spectra are well mod- eled by the canonical description of Galactic black holes in the high-state, a black hole with a steep power law, but whose bolometric luminosities exceed 2 x lo3’ erg s-’, ranging up to 1041.5ergs-’ and whose blackbody temperatures are less than 0.3 keV. If these objects are radiating at M 1/2 the Eddington limit like their Milky Way counterparts their implied masses are from 30 - 3000 Ma, a range very similar to that implied by the low-state objects. Using the M-1/4 scaling of mass to temperature, the observed spectral temperatures give masses of 500 - lo4 Ma a considerably larger value. In general agreement with the expectations of the IMBH hypothesis, the objects with high-state spectra are more luminous than those with low-state spectra. We conclude, from an X-ray spectral and luminosity point of view, that our data are consistent with many of these objects having the properties expected of an IMBH population. However, we also find two other populations of objects, those whose blackbody temperature and luminosity correspond to that of stellar mass black holes with kT M 1keV and log L,y less than 2 x lo3’ ergs-’ and a small population of objects whose X-ray spectra and luminosities are consistent with that of stellar mass black holes in the very high state. Thus, ULX selected purely on the basis of 0.3 - 10 keV X-ray luminosities are a composite class with M 1/4 being ‘(normal” stellar mass black holes and the rest being consistent with a population of IMBHs.
The existence of a substantial population of ULXs in nearby dwarf and other low star formation rate galaxies argues that (in agreement with Ptak & Colbert (2004); Swartz et al. (2004)) there is more than one source term for the origin of ULXs, with at least some of them not being associated with recent star formation, at least statistically. We note that these results have required the high signal to noise of XMM in order to discern the spectrum of these objects. Many of these objects have also been observed by Chandra and their spectra have been well-fitted by simple power laws. In a follow-up paper we will discuss the environments of these objects as revealed by XMM OM UV imaging and the implications this has for the origin of ULXs. I
- 13 -
L.W. gratefully acknowledges Kip Kuntz and M. Coleman Miller for helpful discussions. - 14 -
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This preprint was prepared with the AAS LAWmacros v5 2
L - 17 -
A. Appendix material
The following sources were not best fit by the standard models employed in this study:
A.l. NGC3OO XMM4
This source was classified as a super-soft X-ray source by Kong & DiStefano (2003). We find that the standard single-component absorbed blackbody model is a much better model for this spectrum. In fact, the power law, bremsstrahlung, and combined models do not fit the data within the 90% confidence range. Fitting an absorbed blackbody, we find the best fit corresponds to the following parameters: nH = 1.38t::g; x lo2' cm-2, IcT = 0.059?::::,7, and x2/dof = 74.5/45. This fit yields an unabsorbed flux of 3.3 x ergcm-2 s-l.
A.2. NGC4631 XMM4
The spectrum of this source clearly identifies it as a super-soft X-ray source. As with NGC3OO XMM4, the standard models employed in this study did not adequately match the data. The best fitting model corresponds again to an absorbed blackbody. The corresponding parameters are as follows: nH = 6.22y:g6 x 1021cm-2, IcT = 0.07+::::, and X2/dof = 142.3/74. This fit yields an unabsorbed flux of 9.5 x erg cm-2 s-l . The position of this source shows it to be coincident with a globular cluster associated with that galaxy. This source was identified as a bulge X-ray source, possibly powered by accretion, in a ROSAT study of NGC4631 (Vogler & Pietsch 1996).
A.3. NGC4631 XMM5
The spectrum of this source was best fit with an absorbed power law + an absorbed vupec model. This indicates the prescence of hot gas but we have no further explanation.
A.4. NGC4945 XMM5
The spectrum of this source was not adequately fit with any of the standard models used in this investigation. The spectrum exhibits a prominent Fe K line in the PN spectrum that is well fit by a gaussian (zguuss) at 6.4 keV. We find that the entire spectrum is best fit with a partial covering fraction absorption model (pcfubs) in combination with the normal absorption, a power law, and a gaussian. The best fit parameters yield: absorption column density, nH = 1.79 x 1021cm-2, partial covering absorption, nH = 18.4 x 1021cm-2, partial covering fraction = 0.82, I' = 1.6, and x2/dof = 61.8/57. The source is clearly located within the optical galaxy, and is thus unlikely to -- 18 - be a background AGN, but we have no other explanation.
A.5. M51 XMM5
The spectrum and luminosity (Lx z 1.9 x ergs-') of this source suggests that it is an AGN. The location of the source, from the Digital Sky Survey, places it within the dwarf companion of M51 making a value of the optical flux hard to constrain. The best fit to this source was an absorbed blackbody + power law and the spectral parameters are listed in Table 4.
A.6. M83 XMM2
Like NGC4945 XMM5, this source was best fit by a partial absorption model. However, this source showed no evidence of an Fe K line. We fit this source's spectra using a partial covering fraction absorption model in combination with the normal absorption model and a power law. The best fit parameters yield: absorption column density, nH = 2.1 x 1021cm-', partial covering absorption, nH = 43.5 x 1021cm-2, partial covering fraction = 0.86, r = 2.95, and X'/dof = 83.5/84. The unabsorbed flux in the range of 0.3-10 keV equals 1.37 x ergcm-2 s-'.
A.7. Inverse Compton Scattering Sources
Table 7 includes the parameters for the "ULX" sources best fit by the compST model. A discussion of these sources and interpretation of the data is included in section 5.2. - 19 -
Table 1. XMM-Newton Galaxy Observations
Galaxy Typea nHb distanceC ref obs idd duration (s) comments
NGC247 SAB(s)d 1.54 3.09 0110990301 14536 NGC253 SAB(s)c;HII 1.40 3.73 0110900101, 0152020101 30711, 110591 Starburst NGC300 SA(s)d 3.11 2.56 ... 0112800101 43967 NGC625 SB(s)m? sp; HI1 2.15 2.62 ... 0085100101 26288 NGC1313 SB(s)d; HI1 4.0 4.17 ... 0106860101 41310 IC0342 SAB(rs)cd; HI1 30.3 3.9 1 0093640901 11217 NGC1569 IBm 21.7 1.6 1 0112290801 15582 Starburst NGC1705 SAO- pec; HI1 3.9 5.1 2 0148650101 58926 Starburst MRK 71 BCD; HI1 3.9 3.4 3 0141150201 45919 galaxy pair NGC2403 SAB(s)cd; HI1 4.15 3.56 ... 0150651201 11415 Holmberg I1 Im 3.42 2.70 ... 0112520701, 0112520901 13528, 6860 Holmberg I I AB (s)m 3.49 3.6 4 0026340101 26280 M81 SA(s)ab;LINER 4.12 3.6 4 0111800101 127913 Hol IX also in field of view M82 IO; HI1 4.14 3.9 5 0112290201 29387 Starburst Holmberg IX Im 4.0 3.6 4 0112521001 10330 M81 also in field of view Sextans A IBm 3.85 1.4 6 0026340201 21618 IC 2574 SAB (s)m 2.29 3.6 7 0026340301 24263 bursting star-formation NGC 4214 IAB(s)m; HI1 1.49 2.7 ... 0035940201 14744 NGC 4258 SAB (s)bc;LINER 1.2 7.2 ... 0053140901, 0110920101 16146, 21895 NGC4395 SA(s)m;LINER 1.33 4 ... 0112521901 15842 NGC4449 IBm; HI1 1.39 3.08 ... 0112521701 15522 NGC4490 SB(s)d 1.78 7.8 1 0112280201 17754 interacting with NGC4485 NGC4631 SB(s)d 1.28 7.5 1 0110900201 53850 NGC4736 ( R)SA (r)ab ;LINER 1.43 4.3 1 0094360601 23461 NGC4945 SB(s)cd; Sy2 15.9 3.1 ... 0112310301 23062 NGC 5204 SA(s)m; HI1 1.42 4.8 1 0142770101, 0142770301 19205, 16387 M51 sc; sy2 1.55 7.2 1 0112840201 20924 Galaxy pair M83 S AB (s)c; HI1 3.94 6.2 ... 0110910201 30627 Starburst NGC5253 Im pec;HII 3.77 3.2 1 0035940301 47216 Starburst MlOl SA B (rs)cd 1.17 7.4 8 0104260101 43019 NGC5408 IB(s)m; HI1 5.73 4.8 9 0112290601 7757 Circinus SA(s)b; Sy2 57.8 4 10 0111240101 I10496
afrom the NASA/IPAC Extragalactic Database (NED) bcolumn density in units of 10’’ cm-’, obtained from the web version of the nH FTOOL Cdistancc in Mpc (if no reference is given, obtained from the distance modulus given in LED.4) dXMM-Newtonobservation ids for the data examined in this survey
Referencps. - (1)Tully 1988; (2) Tosi et al. 2001; (3) Tolstoy et al. 1995; (4) Freedman et al. 1994; (5) Sakai & Madore 1999; (6) Sakai, Madore, & Freedman 1996; (7)Shapley, Fabbiano, & Eskridge 2001; (8) Kelson 1996; (9) Karachentsev et a]. 2002; (10) Freeman et al. 1977. - 20 -
Table 2. XMM-Newton best fit: single component spectral fits
Powerlaw Bremsstrahlung
Source nH a r X2/dof .n~~kT (keV) X2/dof Fxb Lx'
NGC247 XMM2 2.29+::0,; 47.7/54 0.54f!$''g 2.55f::i: 48.8154 0.33 0.38 NGC 253 XMM2 (obs 1) 2.51+;::: 69.1/74 o.5+~.'~-0.3 2.12:;:;; 74.7174 0.52 0.87 NGC300 XMM4d 9.07 90.6145 0.27 0.14 117.6145 NGC1313 XMM4 1.82t:yz 141.7/149 140.11149 0.33 0.69 IC0342 XMMl 1.682:::; 159.5/185 1601185 3.5 6.37 IC0342 XMM2 1.852:;;; 77.5/85 74.9185 4.64 8.44 IC0342 XMM4 2.022:::; 64/58 56.9/58 0.69 1.26 NGC2403 XMM4 1.89":::2 62.3/71 62.3171 0.31 0.48 Holm11 XMMl (obs 2) 3.092:::; 266.7/252 309.41252 3.5 3.1 Holm I XMM2 0.35e 2.13+:::: 39.2/45 51.6145 0.10 0.16 Holm I XMM3 0.35e 2.052::;; 34.4/32 42.1132 0.12 0.19 IC2574 XMMl 1.3+:.;: 1.972;:y: 120.9/103 107.5/103 0.35 0.47 IC2574 XMM2 0.4 2;:; 2,2+0.21-0.09 45.7/51 57/51 0.22 0.34 1,1+0.52 44.5138 0.25 0.22 NGC4214 XMMl -0.47 1.872::;; 41.9/38 NGC4258 XMM2 (obs 2) 6.7::-: 2.492;::; 83.6/57 85.5157 0.30 1.9 1.4+0.69 41.3137 0.20 1.2 NGC4258 XMMJ -0.64 2.32:::;; 38.9/37 ... 3.8 1.82 4/11 5/11 0.077 0.48 NGC4258 XMM4 1.972:::; 41.1/48 45.2148 0.39 2.4 2,24+0.29 ... -0.24 77-03/77 77.8177 0.33 2.0 NGC4395 XMM2 2.751::;; 38.6/36 50/36 0.15 0.28 NGC4395 XMM4 0.3'::: 2.08:::339, 16/25 19.9125 0.15 0.28 NGC4449 XMMl 6.32::; 2.2220,:i; 103/ 118 114.31118 1.2 1.36 +0.3 NGC4449 XMM2 1 2.81200::; 103.51112 112.1/112 0.29 0.33 2,09+0.23 NGC4490 XMM4 10.2y; -0.19 51.6/50 50.3150 0.84 6.1 NGC4490 XMh45 3.9+:.;; 2.31+::;; 60.1/65 61.6165 0.41 2.98 NGC4631 XMMlld 7.8 9.50 261.5174 207.8174 NGC4631 XMM5' 1.3 1.03 641.81153 1.3 199 6591153 NGC5204 XMM2 0.89+::2: 1.982::;; 42.37/42 42.2142 0.15 0.41 ... 0.75+0.45 39.4147 0.25 0.69 -0.45 1.63+;:70, 41.4/47 l.lfO.30 2.67+O.2O 140.4182 0.34 2.8 M51 XMMl -0.27 -0.16 110*5/82 0,6+0.30 69.2172 0.18 1.1 M51 XMMJ -0.40 1.86:t:yg 63.2/72 o,4+~.2~ 34.8137 0.16 0.99 M51 XMM4 -0.30 1.55+:.:: 34.8/37
atotal column density in units of 10" cm-2 bunabsorbed flux in the 0.3-10 keV band in units of 10-l' ergcm-2 s-l 'unabsorbed luminosity in the 0.3-10 keV band, using the distances quoted in Table 1, in units of lo3' ergs-' dsee appendix; super-soft X-ray source best fit by single-component blackbody eabsorbtion column density fixed to the galactic column density found in Table 1 'source is best fit by a combined power law and vupec model; see appendix gquestion mark denotes an error in fitting the parameter - 21 -
Table 3. XMM-Newton best fit: two-component blackbody and power law spectral fits
Source nHa kT (keV) r X2/dof Fxc L,yd
NGC247 XMMl 4.11;:;f0.4 0.12f:::i 4.18+;':; 86.5/93 25.7 6.2 7.1 NGC253 XMMl 2.7-,., 0.80'::;; 1.74::::: 225.9/230 36.7 2.7 4.5 ... 7.32;:: 1.142:::; 2.54:;:;; 567/580 44.6 3.4 5.7 NGC253 XMM2 (obs 2) 2.02;:3, 0.712:::: 2.14:::;; 460.3/498 47.1 1.6 2.7 NGC253 XMMJ 3.12;:; 0.75::::; 2.472;::; 68.5/82 23.4 0.60 1.0 ... 3.2:;:; 0.671::;; 2.07::::; 347.4/407 34.4 0.80 1.3 NGC253 XMM4 202;0i8 0.11~:::~ 2.51f::;g 66.7/57 6.9 15 25 ... 4.52::; 0.09'",:",; 2.332;:;; 309.3/291 12.1 1.4 2.2 NGC253 XMM5 1.5-1.5f7.2 0.961::;; 2.432;::; 26.5/23 5.3 0.26 0.43 ... 4.62::; 0.162:::; 1.95'+",:; 223.7/296 60.1 1.4 2.2 NGC253 XMMG 6.32;:: 0.12':::; 2.26::::; 417.9/407 17.1 1.9 3.1 NGC253 XMM7 6.327:; 0.692:::; 2.402::;; 335.8/339 21.2 1.1 1.8 NGC300 XMMl 1.7f;::: 0.982:::: 3.412::;: 443.7/420 26.1 1.3 1.0 NGC300 XMM2 3.8:::: 0.092:::: 2.872:::; 102.6/97 31.34. 1.1 0.86 NGC300 XMM3 4.41;:; 0.04::::; 1.98'8:: 87.7/79 14.2 1.2 0.93 NGC3OO XMMG 2.3-,.,f2.6 0.84?::$ 4.9?:::7 34.6/35 13 0.27 0.20 NGC1313 XMMl 3.05;:; 0.13f:::z 1.75+0.14-o,ll 194.1/201 35.4 0.64 1.3 NGC1313 XMM2 3.11::; 0.162:::; 2.272:::; 425.2/419 38.9 2.0 4.2 NGC1313 XMM3 6.2:;:: 0.11::::; 2.76+:::7 441.7/424 336.6 10 22 IC0342 XMM3 9.7-+'.' 2.1 0.09':::; 2.69+::;: 129.5/107 56.3 31 56.4 NGC1705 XMMl 0.29f:::: 1.012::;; 2.312:::; 53/85 8.9 0.10 0.14 NGC1705 XMM2 0.96:::;; 0.232:::; 1.602:::: 85.5/74 6.5 0.09 0.27 NGC1705 XMM3 0.93+!51e 1.07+::1: 2.23::::; 69.8/65 11.1 0.15 0.48 MRK71 XMMl 1.4'::: 1.45:::;; 2.572A.i; 52.6/54 4.1 0.24 0.33 NGC2403 XMMl 2.31;:; 0.66':;:; 2.18+:::: 81.4/79 10.8 1.99 3.1 NGC2403 XMM2 1.82;:; 0.622:::; 1.95+0.'6-o,42 163.1/151 16.4 1.0 1.6 NGC2403 XMM3 1.72;:; 0.74:::;; 2.15+:,!; 84.2/105 8.4 0.64 1.1 Holm I1 XMMl(obs 1) 1.62;:; 0.14+:,:; 2.35'::;: 997.5/976 136.7 12 10 Holm I XMMl 0.4-,,,+0.5 1.972::;; 2.461:::40 97.4/93 5.4 0.6 0.93 M81 XMMl 3.62:::: 0.912:::; 2.701:-:2 1316.8/1243 533.1 5.0 7.8 ... 3.52::; 1.13::::; 2.342::;; 203.5/204 21.4 4.8 7.4 M81 XMM2 7.42::; 0.1-o,oo4+0.004 2.87::::; 833.9/616 524.3 13 22 3.7-,,,f2.4 0.112:::; 1.69+0.27 M81 XMMJ -0.33 77.1/78 4.25 0.53 0.82 M81 XMM4 1.12;:; 2.512;::; 2.312::;; 48.9/50 28.2 0.43 0.70 M81 XMM5 0.15'::;; 0.62::::; 1.262::;; 89/80 8.5 0.38 0.59 Holm IX XMMl 2.lf::i 0.17-0,02+0.02 1.721:::; 866.6/878 134.3 10 16
NGC4214 XMM2 1.8?;:: 0.81f::i; 3.952i.805 46.4/44 4.5 0.4 0.35 NGC4258 XMMl 0.382:::6 0.542::;; 1.512::: 91.1/76 10.3 0.34 2.1 NGC4258 XMM2 (obs 1) 1.9?;:: 0.78t:::; 2.0217:;5 73.4/61 24.1 0.31 1.9 NGC4395 XMMl 2.0':::~ 0.14::::; 3.44+0.j4-o.56 168.2/154 26.9 1.4 2.7 NGC4395 XMM3 0.5f::' 1.102:;;; 2.66+'.05-0.77 52/56 3.9 0.29 0.56 NGC4449 XMMJ 3.52;:; 0.15?:::3 2.52'::;; 119.9/87 34.1 1.1 1.3 NGC4490 XMMl 5.8-2.96+2.96 0.77?:::i5 2.892::;: 66.5/63 35 0.88 6.4 NGC4490 XMM2 4.42::; 0.60':::: 2.132::;: 42.4/54 7.1 0.65 4.7 NGC4490 XMM3 13f9.6-2.5 0.09':::; 3.21+0.j2-0.17 72.1178 4.6 12 87.4 3fO.9 NGC4631 XMMl -0.5 0.125:;:; 2.12+:::: 371.3/345 12.1 0.96 6.5 - 22 -
Table 3-Continued
Source nHa kT (keV) r X2/dof Axzb Fxc LX
NGC4631 XMM2 1.80:",;: 107.4/97 12.1 0.25 1.7 NGC4631 XMM3 2.452;,,, 127.1/96 18.9 0.15 1.o NGC4945 XMMl 1.60+::;: 96.1/120 20 0.59 0.68 NGC4945 XMM2 1.802::2,: 105.8/113 8.7 0.66 0.76 NGC4945 XMM4 2.822;::; 58.4/60 17.1 0.38 0.44 NGC5204 XMMl 2.962:::; 551.1/559 70.3 3.0 8.3 3.252::;; 566.8/496 60.1 5.3 15.0
~ M5lXMM2 1.80f::$ 70.7/68 4.5 0.36 3.0 M51 XMM5 2.26+::;: 59.8/70 196.2 220 1900 M51 XMM7 1.972;:;; 31.7/29 6.1 0.26 1.6 M83 XMMl 2.22+::33: 206.1/209 4.9 0.53 2.5 M83 XMM4 1.52:;;;; 95.1/89 12.4 0.2 0.92 MlOl XMMl 1.422;:;; 249.9/231 53.1 0.45 2.9 MlOl XMM2 1.88+:::: 251.6/261 37.2 0.7 4.6 MlOl XMM4 2.222:::; 1%.2/ 138 7.5 0.34 2.2 NGC5408 XMMl 2.71f:::; 316.41337 80.4 3.97 10.9 CIRCINUS XMMl 2.302;::88 749.4/861 13.5 12 23 CIRCINUS XMM2 4.711:::; 438.5/430 79.4 5.6 10.7 CIRCINUS XMM3 5.77f;:Y 269.3/260 15.9 7.6 14.5
atotal column density in units of loz1 cm-2 bimprovement in x2 over the single-component power law model Cunabsorbed flux in the 0.3-10 keV band in units of erg cm-2 s-l dunabsorbed luminosity in the 0.3-10 keV band, using the distances quoted in Table 1, in units of erg s-l equestion mark denotes an error in fitting the parameter - 23 -
Table 4. XMM-Newton best fit single/two-component spectral fits
power law blackbody and power law
Source nHa r x2/dof nHa kT (keV) r x2/dof FxC Lxd
NGC300 XMM5 ?';e 2.13:;; 49.3/55 0.41f::;: 1.06'~:~~ 2.782::;: 47.3/53 0.17 0.13 Sextans A XMMl 0.18+0.23-o,16 2.252:::; 271.4/275 0.4+::: 1.05z$i7 2-62::; 269.1/271 0.60 0.14 IC2574 XMM3 0.152:::; 2.43z:::L 40.3/49 1.1':;; 0.87+::$: 3.56::::; 38.4/47 0.36 0.56 NGC4736 XMMl 0.951::; 2.02+0.26-0.25 62.8153 6.32;:: O.OSz",$ 2.412::;; 54.9/51 8.1 17.9 NGC4945 XMM3 3.3:;:; 1.82+u.12-0.20 30.8/30 6.8:::; O.ll?:::g 2.03':::; 29.7/28 0.95 1.09 M51 XMM6 2.02;:;; 2.50?::;: 40.97/43 8.2';:; 0.08':::; ~.o+O.~'-o.43 36.9/41 5.6 35 MlOl XMM3 1.5:::; 2.70':::; 130.3/133 1.62::: 0.64+:::: 2.932::; 129.9/131 0.56 3.7 +1.2 +0.3 MlOl XMM5 1.32;:: 2.28'::;: 47.9/46 1.3-0,2 0.18':::; 1.95-o,22 45.1/44 0.13 0.85 atotaI column density in units of lo2' cmp2 bimprovement in x2 over the single-component power law model Cunabsorbed flux in the 0.3-10 keV band in units of erg cm-2 s-l for combined fit dunabsorbed luminosity in the 0.3-10 keV band, using the distances quoted in Table 1, in units of erg s-l gquestion mark denotes an error in fitting the parameter - 24 -
Table 5. Bolometric Luminosities of ULX sources
Source LILPZ)~+' Lbol MEddC
NGC247 XMMl 13.4258 7.07734 54.4411 NGC253 XMMl 9.31469 2.44574 18.8134 NGC253 XMM2 4.3701 2.15292 16.561 NGC253 XMMG 5.05828 3.92514 30.1934 NGC1313 XMM3 37.0364 27.9692 2 15:148 NGC1313 XMM4 1.50345 ... 115.65 IC0342 XMMl 14.1215 ... 1086.27 IC0342 XMM2 19.8129 ... 1524.07 IC0342 XMM3 114.015 95.4068 733.899 NGC2403 XMMl 4.1497 2.14873 16.528 NGC2403 XMM4 0.57068 ... 43.898 Holmberg I1 XMMl 0.88906 ... 68.3893 ... 16.8335 11.4543 88.1103 Holmberg I XMMZ 10.5158 ... 808.908 M81 XMMl 15.7004 3.17932 24.4563 Holmberg IX XMMl 31.0582 28.1445 216.496 NGC4214 XMMl 0.26699 ... 20.5379 NGC4258 XMMS 0.46503 . . . 35.7715 NGC4395 XMMl 9.04609 2.94683 22.6679 NGC4449 XMMl 2.11312 ... 162.547 NGC4449 XMM2 2.48586 . . . 191.22 NGC4490 XMMl 16.8513 3.21972 24.7671 NGC4490 XMM2 7.36612 4.51554 34.7349 NGC4490 XMM3 240.653 176.04 1354.15 NGC4490 XMM4 1.66829 . . . 128.33 NGC4490 XMMS 12.7136 . . . 977.971 NGC4631 XMMl 10.6527 8.59661 66.1278 NGC4736 XMMl 31.6561 27.3664 210.511 NGC5204 XMMl 22.4756 2.20492 16.9609 NGC5204 XMM2 5.57769 . . . 429.053 M51 XMMl 4.76208 ... 366.314 M51 XMM2 4.25898 3.57502 27.5001 M5l XMM3 2.10133 . . . 161.641 M51 XMM4 2.56064 ... 196.972 M51 XMMG 46.7642 39.5189 303.991 MlOl XMMl 8.04916 7.68224 59.0942 MlOl XMM2 7.54268 4.96709 38.2084 MlOl XMM3 10.9792 1.03659 7.97381 NGC5408 XMMl 20.9211 11.5369 88.7455 Circinus XMMl 70.3579 56.1033 431.564 Circinus XMMZ 208.746 0.69929 5.37916 Circinus XMM3 771.157 0.212957 1.63813
=upper limit on the bolometric luminosity, determined with an exponential cut-off in the power law at high energy (see text) bbolometric luminosity estimate for high-state ULXs where the p.ower law is cut at twice kT;, (see text); units - 25 -
for both luminosity measurements in lo3’ erg s-l
‘mass computed for objects radiating at 0.1 x LE& (low- state objects) or LEdd (high-state objects; using &,), in units of Ma - 26 -
E. 2
2 Kim
3
09-703 mmm oo?) ***P-r-t. 000 000 - 27 -
2
3 X
m ro 0 5
.- 9 X
I W
-!c?"c? ?1c9 ?@?Nu? Nom0 -?om* +NO0 0-m~mv-m doom m2 wmmm NW*O imom mmme 3000 00-10 0h mmmo. *--m. mmmm wwwr-. t-t-r-w . wwww r- ++++.++++.++++ d+
c! m N 2 N 3 5 s X xxxxx m mN S 0 7, - - I 28
2
t- 4
0W t-r- 9 3 4 0 ??." 2 E 3 a .^ m 4 -m -Jcv) 0 +
23 m 0 e4 T 3 0 X u
1 m x 2 3
r- 3 q 3 n- se 0-m hliD
10 3 m 4 .-.In.+
m 0s
4 - 29 -
omm-a (0 w- WWb I ***m34X mmm-+++a
ii) 3 - 30 -
m -m V e m c,
m u 3 8 - 31 -
Table 7. Bright, Identifiable Background and Foreground Sources
Galaxy RA (h m s) Dec (0 I I/) Identification
NGC 247 0 46 51.7 -20 43 30 QSO B044-2059 NGC 300 0 55 26.7 -37 31 25.6 HD 5403'(Star) NGC 625 01 34 42.4 -41 36 15.2 QSO B0132-4151 NGC 156ga 04 31 16.9 $64 49 50 CXOU 5043116.8$644950 (Star) NGC 1569 04 31 14.2 $64 51 07.9 CXOU 043114.0+645107 (Star) NGC 1569 04 31 25.4 $64 51 53.8 CXOU 043125.1+645154 (AGN) NGC 1705 04 54 01.2 -53 21 12.3 WGA JO454.0-5320 (M star or elliptical galaxy) NGC 2403 07 35 09 $65 40 27.5 HD 59581 (Star) NGC 4258 12 18 08.9 $47 16 08.3 QSO J1218+472 M83 13 36 45.6 -29 59 13.9 2MASX 513364579-2959122 (Galaxy) M83 13 36 13.9 -29 56 13 RX 5133615-2957.8 (Galaxy) NGC 5253 13 39 50.6 -31 34 11.1 CD-30 10790 (Star) MlOl 14 02 30 $54 21 18.2 [WIP99] H13 (Star)b NGC 5408 14 03 27.5 -41 25 18.5 (Star)
aidentification for objects in NGC 1569 from Martin, Kobulnicky, & Heckman (2002) bconfirmed by K. Kuntz using HST ACS - 32 -
Table 8. Spectral Simulations
ID exposure time (s) power law x2 bremsstrahlung xz blackbody and power law x2
10% exposure time
~~ NGC 247 XMMl 1130 96.4194 223.75194 72.7191 NGC 253 XMMl 10778 655.21581 667.81581 648.71578 NGC 1313 XMMl 2789 188.71201 189.081201 187.21197 NGC 1313 XMM2 2788 425.71419 431.71419 423.51415 IC0342 XMM1 311 164.41183 208.71183 1261179 IC0342 XMM2 511 236.41107 189.91107 160.91103 Holmberg I1 XMMl 982 1034.61976 1152.11976 971.21972 M81 XMMl 9082 1314.411244 2010.911244 1246.1/1241 Holmberg IX XMMl 709 898.71882 910.71882 881.81878 Circinus XMM2 9005 395.31431 388.71431 385.61428
5% exposure time
NGC 247 XMMl 565 107.9194 146.4194 77/91 NGC 253 XMMl 5389 516.31581 519.21581 516.31578 NGC 1313 XMMl 1394 125.81201 125.91201 122.91197 NGC 1313 XMM2 1394 273.31419 273.31419 263.31415 IC0342 XMMl 256 141.11183 157.71183 125.51179 IC0342 XMM2 256 122.61107 99.81107 1481103 Holmberg I1 XMMl 491 908.31976 981.81976 871.31972 M81 XMMl 4541 1086.811244 1426.211244 1060.7/1241 Holmberg IX XMMl 354 663.61882 635.91882 629.21878 Circinus XMM2 4502 258.71431 256.91431 255.11428 - 33 -
Table 9. XMM-Newton power law fit for best fit two-component spectra
Source nHa r x2/dof FxC Lxd
NGC247 XMMl 9.5:;:: 112.2195 1900 2200 NGC253 XMMl 3.42;:; 262.61232 3.1 3.6 ... 6.9'::; 611.61582 2.9 3.3 NGC253 XMMZ (obs 2) 2.22::; 507.41500 1.2 1.4 +0.5 NGC253 XMM3 3.9-~.4 91.9183 0.73 1.2 ... +0.4 4.0-o.3 381.81409 0.98 1.6 NGC253 XMM4 8.5'?: 73.6159 0.52 0.85 ... 1.2:;:; 321.41293 0.29 0.48 NGC253 XMMS 1.72;:; 31.8125 0.32 0.53 ... 3.4:;:; 283.81298 1.1 1.3 NGC253 XMMG 3.92::; 4351409 0.93 1.5 f0.7 NGC253 XMM7 7.1-~.7 3571342 1.3 2.2 NGC300 XMMl 0.97':::: 469.81422 0.81 0.6 NGC300 XMM2 1.7f0.40 133.98199 0.27 0.21 NGC3OO XMM3 ,,,i8:Q0 101.9181 0.19 0.15 NGC3OO XMMG ?+?0.6- 47.6137 0.06 0.15 NGC1313 XMMl 1.5+::; 219.81203 0.42 0.88 2.8f0.16 NGC1313 XMM2 -0.16 464.11421 1.9 4.0 f0.2 NGC1313 XMMS 3.6-0.2 778.31426 3.3 6.9 IC0342 XMMS 3.82::; 185.81109 1.7 3.1 NCC1705 XMMl 0.3e 61.9188 0.12 0.37 1.4fO.45 NGC1705 XMM2 -0.41 91/76 0.078 0.24 0.6f0.36 NGC1705 XMM3 -0.40 80.9167 0.17 0.53 3.2+0.61 NGC2403 XMMl -0.55 2.15+:::; 92.2181 2.2 3.6 2.7-bO.37 NGC2403 XMM2 -0.34 2.07:::;; 179.51151 1.3 2.0 1.gf0.40 NGC2403 XMM3 -0.36 1.97:::;; 92.61107 0.81 1.3 Holm11 XMM1 (obs 1) 1.5fo.07-0.07 2.61f::;; 1134.21976 12 10 ?+? Holm I XMMl -? 2.042;:;; 102.8195 0.48 1.7 3.2f0.07 M81 XMMl -0.07 2.09:;:;; 1849.911245 4.5 7.0 ... 3.0+::: 1.79::::; 224.91208 4.3 6.7 M81 XMMS 7.3 6.13 1358.21618 48.5 75.2 0.97+O.Z M81 XMMJ -0.41 1.582:::: 81.35180 2.5 3.9 ?+? M81 XMM4 -? 0.882:::: 66.4152 0.33 0.54 M81 XMM5 1.0:::; 1.52::::: 97.5182 0.44 0.68 IX XMMl f0.08 Holm 1.7- 0.08 1.84+::00; 1000.9/882 9.4 15 NGC4214 XMM2 0.25; 2.031::;; 50.9146 0.16 0.14 NGC4258 XMMl 1.62;:; 101.4178 0.06 0.04 NGC4258 XMM2 (obs 1) 3.5:;;: 97.5163 0.43 2.6 f0.5 NGC4395 XMMl 3.7-o.4 195.11156 7.2 14 ?+? NGC4395 XMM3 -? 55.9158 0.25 0.48 NGC4449 XMM3 3.3:;:; 154189 1.3 1.5 iXGC4490 XMMl 0.83:::;; 101.5165 1.2 8.7 NGC4490 XMMZ 6.31;:; 49.5156 0.92 6.7 fl.5 NGC4490 XMM3 9.4-12 76.7180 1.5 11 2.3+0.16 NGC4631 XMMl -0.15 383.41347 0.76 5.1 NGC4631 XMM2 1.9r;:; 119.5199 0.23 1.5 - 34 -
Table 9-Continued
Source nHa r X2/dof Fxc Lxd
NGC4631 XMM3 0.632::; 1.53+& 146198 0.15 1.0 NGC4945 XMMl 5.82::; 1.88+;::8, 1161122 0.9 1.0 NGC4945 XMM2 3.42::; 1.582:::; 114.51115 0.71 0.82 NGC4945 XMM4 5.2:::; 2.592:::; 75.5/62 0.49 0.56 NGC5204 XMMl 0.61::;; 2.11+;::: 621.4/561 2.0 5.5 ... 1.1:;:; 2.41+::;; 626.9/498 3.0 8.3 2,3+0.50 3.3 M51 XMM2 -0.30 2.502::;; 75.2/70 0.52 M51 XMh.15 2.7 3.08 256.0172 0.43 2.7 0.5f0.39 0.11 0.66 M51 XMM7 -0.46 37.8/31 1.g-kO.34 M83 XhfMl -0.31 210.91211 0.64 2.8 M83 XMM4 4.45:. 107.5/91 0.4 1.8 MlOl XMMl 0.56+::;: 3031233 0.45 2.9 2.24-0.25 MlOl XMM2 -0.23 288.81263 0.81 5.3 2.2W.45 MlOl XMM4 -0.42 165.71140 0.38 2.5 NGC5408 XMMl 1.6'",: 396.8/339 7.04 19.4 CIRCINUS XMMl 7.62:,: 762.91863 4.6 8.8 CIRCINUS XMM2 11.7+::$ 517.91432 2.7 5.2 CIRCINUS XMM3 9.0';:; 285.21262 0.32 0.61
atotal column density in units of loz1 cm-2 bimprovement in x2 over the single-component power law model Cunabsorbed flux in the 0.3-10 keV band in units of lo-" erg cm-' s-' dunabsorbed luminosity in the 0.3-10 keV band, using the distances quoted in Table 1, in units of erg s-l eabsorption level frozen at approximate galactic level - 35 -
Table 10. Best-Fit Absorbed Comptonization Model Parameters
ID nHa kTb tauC X2 FXd
NGC 253 XMM2 1.82:::; 1.28::::; 19.592::; 464/498 1.47 NGC 2403 XMMl 1.95t::i 0.982::;: 25.42::; 82.8/85 1.4 NGC 4490 XMMl 4.72::: 0.962:::3, 27.02:8i3 66.5/64 0.66 NGC 4490 XMM2 5.0:;;; 1.21:::;; 18.8+$ 45.7/55 0.67 MlOl XMM2 1.62:;;; 1.24200::: 23.3:;:; 256/262 0.65 MI01 XMM3 1.12:::; 1.13 15.2 i2a/i32 0.41 Circinus XMM2 6.82;:; 0.622:::; 29.72;:; 437.2/430 0.5 Circinus XMM3 6.72::; 0.932::2,: 23. l::7i5 273.1/261 0.17 -
atotal column density in units of loz1 cm-2 btemperature in keV Copticaldepth dunabsorbed flux in the 0.3-10 keV band in units of erg cm-2 s-l - 36 -
Table 11. XMM-Newton Galaxy Observations
NGC247 7.93 27.32 0.602 0.687 1 NGC253 998.73 1861.67 55.92 93.10 3 NGC3OO 23.08 74.45 1.688 1.324 0 NGC625 5.09 9.08 . 0.280 0.230 0 NGC1313 35.97 92.00 2.329 4.845 2 IC0342 255.96 661.68 16.66 30.32 3 NGC1569 45.41 47.29 2.072 0.635 0 NGC1705 0.970 2.580 0.064 0.199 0 MRK 71 3.51 4.67 0.173 0.239 0 NGC2403 51.55 148.49 3.547 5.378 2 Holrnberg I1 1.15 2.62 0.070 0.061 1 Holrnberg I ...... 1 M81 44.73 174.02 3.647 5.655 1 M82 1271.32 1351.09 58.35 106.2 1 Holmberg IX ...... 1 Sextans A 0.255 0.6’74 0.017 0.004 0 IC 2574 2.41 10.62 0.212 0.329 0 NGC 4214 17.87 29.04 0.947 0.826 1 NGC 4258 21’.60 78.39 1.690 10.48 1 NGC4395 4.21 12.90 0.299 0.573 1 NGC4449 37.00 58.28 1.937 2.199 2 NGC4490 47.79 85.94 2.636 19.19 5 NGC4631 82.90 208.66 5.324 35.83 1 NGC4736 62.41 135.34 3.734 8.261 4 NGC4945 588.11 1415.5 36.95 42.49 0 NGC 5204 2.33 5.35 0.143 0.395 2 M5l 108.68 292.08 7.213 44.74 5 PI83 266.03 638.63 16.69 76.79 0 NGC5253 30.00 30.92 1.365 16.72 0 MlOl 88.04 252.84 6.048 39.63 4 NGC.5408 2.825 2.958 0.129 0.356 1 Circinus 248.7 315.85 12.06 23.10 4 aflux in units of 10-~ergcm-2 s-’ bfar-infrared luminosity in units of ergs-l - 37 -
5 1 I I I I I I I I I I I I I. I I
4
rcr 0 L xz E
1
0 so- SO SO+ Sa 3ab Sb Sbc Sc Scd Sd SmIABmIBm Im IO Hubble Type
Fig. 1.- Distribution of galaxies by Hubble type among our archival XMM-Newton sample of nearby (< 8 Mpc) galaxies. Our sample consists solely of spirals and irregulars. - 38 -
c? I I I I I I I I"" I' I I
4
3 rcl 0 L Q) D E 7 z 2
0 I u -2 -1 0 1
Fig. 2.- The distribution of fz/fopt for the brightest possible optical point source within the XMM-Newton error circle. We define fz as the unabsorbed X-ray flux in the 0.3 - 10 keV range and fopt as the optical flux obtained from the U filter of XMMs OM (as described in text). These ratios do not represent the actual fx/fopt of the sources but are an estimate of the maximum possible value. A majority of the sources had no optical point source within the X-ray contour and thus have ratios of fZ/fopt far higher than those indicated in the plot. - 39 -
10 II 1111 1111 1111 Ill1
8
I
3 6 rcr 0 k a, 9 E 4 z7
2 r 0 2 3 4 5 6 Spectral Index (r)
Fig. 3.- Distribution of the spectral indices (I?) for low-state (shaded) and high-state (open) objects. For Galactic low-state objects, typically M 2.0, similar to our sample, while the high- state objects have a steeper r (McClintock & Remillard 2004). - 40 -
A
A
A A A A
w l=
rn
w -
38 IIII IIII Ill1 IIII IIII 1 2 3 4 5 6 Spectral Index (r)
Fig. 4.- Relationship of luminosity vs. spectral index for low-state (rectangle) and high-state (triangle) objects. As expected from observations of Galactic stellar-mass black hole systems (Mc- Clintock & Remillard 2004), the classified low-state ULXs in our sample have, on average, lower X-ray luminosities than the corresponding high-state ULXs. We plot the mean values for both high-state and low-state objects with errorbars indicating the root mean square deviation. The outlying objects with spectral indices greater than 3.5 were not included in the mean or deviation calculations. -41 -
40.5 ixx
Fig. 5.- (1eft)Distributionof the blackbody temperature for high-state objects. (right)Relationship of blackbody temperature vs. luminosity (in the 0.3-10 keV band) for high-state objects. We see two peaks arise in the distribution, one centered around kT= 0.1 and another at kT= 1. The peak with a low disk temperature also corresponds to the highest luminosities, suggesting that these may be high-state IMBHs. The sources with higher disk temperature also have lower luminosities. The spectra of these sources were also well fit, by an inverse Comptonization model (a model succesfully used to fit some of the Galactic black hole X-ray binaries in the very high state). - 42 -
1.5
k 0 -10 u a k a 3=
0.5
0
Fig. 6.- Color-Color Diagram plotting soft vs. hard colors, as outlined in Done & Gierlinski (2003), for low-state (rectangle) and high-state (triangle) ULXs. A large number of our sources lie in the same range of this graph as the black hole sources examined by Done & Gierlinski (2003) (near the power law distribution, indicated by the solid line). The dashed line represents the color- color plot for a multi-colored disk model with different disk temperatures. The sources approaching this line were those well-fit by the Comptonization model. Done & Gierlinski (2003) had no black hole sources in this region, but atolls and Z-sources, which were also well-fit by Comptonization models. - 43 -
Fig. 7.- (left) Relationship of the far-infrared luminosity, as an indicator of star formation rate, vs. the number of ULXs for each galaxy. If ULXs are associated with star formation, we naively expect that the higher the FIR luminosity the more ULXs the galaxy will host. (right) The distribution of average number of ULXs / LF~Rbin for spirals (solid line) follows this expectation. The distribution of irregulars (dashed line) is not so easily interpreted. The numbers at the top indicate the number of spirals/irregulars in each of the luminosity bins. More irregulars would need to be included in this survey for meaningful statistics on this group. - 44 -
4.c I I I I I I I I I I I I I I 1- 1- I 1
Fig. 8.- Distribution of the hydrogen column densities of ULX sources. The nH values were obtained through spectral fits using the wabs model in XSPEC. Galactic column densities towards the host galaxy were subtracted from the spectral fit values. A majority of our ULX sources have high column densities (> 1021cm-2),suggesting that some of this absorption originates with the local ULX environment. Bins to the left of the dashed line represent sources with column densities very close to the Galactic value and thus a simple subtraction is not statistically representative of the true value.